Known in the past has been an exhaust purification system of an internal combustion engine comprising an exhaust purification catalyst arranged in an exhaust passage of an internal combustion engine, an upstream side air-fuel ratio sensor arranged at an upstream side of the exhaust purification catalyst in the direction of exhaust flow and detecting an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst, a downstream side air-fuel ratio sensor arranged at a downstream side of the exhaust purification catalyst in the direction of exhaust flow and detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, and a control apparatus using the output air-fuel ratios of the upstream side air-fuel ratio sensor and the downstream side air-fuel ratio sensor as the basis to control an amount of feed of fuel to a combustion chamber of the internal combustion engine.
In such an exhaust purification system of an internal combustion engine, the control apparatus performs main feedback control controlling by feedback the amount of feed of fuel so that the air-fuel ratio detected by the upstream side air-fuel ratio sensor (below, referred to as an “output air-fuel ratio”) becomes a target air-fuel ratio. In addition, it performs sub feedback control using the output air-fuel ratio of the downstream side air-fuel ratio sensor etc. as the basis to alternately switch the target air-fuel ratio between an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, referred to as a “rich air-fuel ratio”) and an air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, referred to as a “lean air-fuel ratio”) (for example, PLT 1). In particular, in the exhaust purification system described in PLT 1, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio or becomes less, the target air-fuel ratio is switched from a rich air-fuel ratio to a lean air-fuel ratio. In addition, if the estimated value of the oxygen storage amount of the exhaust purification catalyst reaches a switching reference storage amount smaller than a maximum storage amount of oxygen of the exhaust purification catalyst, the target air-fuel ratio is switched from a lean air-fuel ratio to a rich air-fuel ratio.
Further, the output air-fuel ratio of the upstream side air-fuel ratio sensor deviates to the rich side the greater the amount of hydrogen in the exhaust gas discharged from the engine body. Therefore, in the exhaust purification system described in PLT 1, the output air-fuel ratio of the upstream side air-fuel ratio sensor was used as the basis to calculate the amount of release of oxygen from the exhaust purification catalyst in the time when the target air-fuel ratio is set to a rich air-fuel ratio and the amount of storage of oxygen in the exhaust purification catalyst in the time when the target air-fuel ratio is set to a lean air-fuel ratio. Further, a sub learning value is calculated in accordance with a difference between the thus calculated oxygen release amount and oxygen storage amount and this sub learning value is used as the basis to correct the target air-fuel ratio (sub feedback learning control). According to PLT 1, due to this, even if deviation occurs in the output air-fuel ratio of the upstream side air-fuel ratio sensor, it is considered that this deviation can be compensated for.